In commercial rechargeable lithium batteries, lithiated transition metal oxides are employed as a cathode active material and these transition metal oxides include, for example, materials of the layered crystal structure such as LiCoO2, Li(Mn1/2Ni1/2)1-xCoxO2, or LiNi1-xCoxO2, and materials of the spinel crystal structure such as lithium manganese oxide spinel or lithium manganese-nickel oxide spinel. Depending on the application, certain properties of these materials are of importance and such properties can be modified by processing, doping, surface treatment, control of impurities, etc.
Some of these materials, particularly where only one type of transition metal is present, can be easily prepared by solid state reaction using simple transition metal precursors. However, more “complex” materials, particularly where two or more types of transition metals are present, are difficult or impossible to prepare by simple solid state reaction, i.e. by mixing separate transition metal precursors. Instead, complex lithium transition metal oxides are generally prepared by reacting mixed precursors, e.g., mixed hydroxides or mechanically alloyed transition metal oxides, with a source of lithium.
Mixed hydroxides are typically prepared by precipitation reaction. Precipitation of mixed hydroxides (for example, the precipitation of a flow of NaOH with a flow of M-SO4 under controlled pH) or mixed carbonates (for example, the precipitation of a flow of Na2CO3 with a flow of M-SO4) allows precursors of suitable morphology to be achieved. A problem is the level of impurities; especially, the removal of sulfur is difficult and expensive. The same problems apply to the removal of sodium in the case of mixed carbonates.
Meanwhile, as one method for modification of lithium transition metal oxides, doping has been widely investigated. The doping typically does not exceed 5% by atoms of dopant per transition metal. Typical dopants are either inserted isostructurally into the existing crystal structure (e.g., Al-doped LiCoO2) or they form a secondary phase, often agglomerating at grain boundaries (e.g., Zr-doped LiCoO2).
In the doping approach, aluminum is a general dopant. The benefit of aluminum-modified lithium transition metal oxides has been widely investigated. For example, it is known that adding Al to the crystal structure of LiNi1-xCoxO2 improves safety and cycling stability. For example, Al-doped LiMn2-xLixO4 spinel cycles more stably and also shows less dissolution of Mn, and Al-coated LiCoO2 cycles more stably at high voltage.
A problem associated with aluminum-modified lithium transition metal oxides, i.e., Al-containing lithium transition metal oxides, is the preparation process thereof. In the case of complex lithium transition metal oxides, mixed precursors would have to contain aluminum; however, it is more difficult to prepare Al-containing precursors such as Al-doped mixed transition metal hydroxide. Alternatively, lithium transition metal oxides could be prepared by mixing raw materials with a source of aluminum such as Al2O3 or Al(OH)3. In this regard, it should be noted that Al2O3 has low reactivity and Al(OH)3 is easily transformed to Al2O3 at low temperature. Therefore, the obtained cathode is nonhomogeneously doped so that the benefit of aluminum doping is not fully utilized.
In a process for preparation of Al-doped materials, if a layer containing a reactive aluminum phase were to fully cover the surface of a subject particle such as lithium transition metal oxide, this would be advantageous. If this were possible, the diffusion pathway would be short and the contact area would be large so that an Al-doped material could be achieved at relatively low reaction temperature. As will be illustrated later, the present invention discloses such composite precursor fully coated with a reactive aluminum phase and a process for preparation of the composite precursor.
Meanwhile, besides the Al doping approach, an Al coating approach is also known as a means to improve properties. In a conventional Al coating process, lithium transition metal oxide particles are dipped into an aluminum-containing solution or gel, followed by drying and mild heat treatment. As a result, the surface of lithium transition metal oxide is coated by an aluminum oxide-based phase. This phase separates the electrolyte from the more reactive bulk and promises improved properties. However, the conventional Al coating process has demerits as explained below.
In the prior art Al coating process, lithium transitional metal oxides are dipped into AlPO4 or tri-butyl aluminum dissolved in ethanol. Problems are the cost of raw materials and the use of organic solvents that may cause the generation of gas during reaction or drying procedures. A further problem is that only a small amount of Al can be coated on the lithium transition metal oxide. Low solubility of AlPO4 or tri-butyl aluminum limits the amount of aluminum present in a layer formed by dip-coating. Where an organic solvent is used in large amounts to compensate for the low solubility, aluminum-containing particles form, but fail to cover the lithium transition metal oxide. Generally, the contact between the aluminum compound and lithium transition metal oxide after a drying procedure is maintained mainly by physical adhesion and to a lesser extent by chemical bonds. Accordingly, although a thicker coating layer is made, it tends to disintegrate during drying.
As an alternative approach, a particle coating process is also known in the art. In this process, coating is achieved by dipping lithium transition metal oxides into a slurry of fine particles. Alternatively, it is also possible to apply a dry coating approach. In this dry coating process, fine powders, typically Al2O3 particles of sub-micrometer size are mixed with lithium transition metal oxides. However, the particle coating process has some disadvantages, as follows: (i) it is difficult to achieve a full coverage by fine particles; (ii) it is difficult to prevent agglomeration of fine powders, and the resulting agglomerates fail to efficiently cover the surface of lithium transition metal oxide; and (iii) the adhesion between fine particles and lithium transition metal oxide is poor so that the coating layers tend to peel off during subsequent processes.
Therefore, improved precursors for cathode active material and a method to prepare such precursors are needed. The improved precursors could be characterized as lithium transition metal oxide with a uniformly thick layer fully covering the particle, the layer having good mechanical contact, containing aluminum and being practically free of impurities.